Various techniques have been used to improve the bandwidth efficiency and noise immunity of communication channels. One particularly useful technique is rotationally-invariant, multidimensional trellis-coded modulation. Such codes have been disclosed, for example, in U.S. Pat. No. 4,713,817, issued Dec. 15, 1987, entitled "Multidimensional, Convolutionally Coded Communication Systems." However, certain prior art codes had a defect in that the same sequence of signal points can be generated by the trellis encoder when the trellis encoder traverses two different paths of trellis states.
This defect in the prior art is not easily corrected, especially when the code must, at the same time, achieve rotational invariance and a large amount of coding gain.